Recently, from standpoint of saving natural sources and energy, small sizes and high efficiency of electromagnetic or electronic parts have been demanded, and soft magnetic property, especially Si steel sheets having excellent iron loss have been also required. It is known that soft magnetic properties of Si steel sheets are improved with increasing of addition of Si and exhibit the maximum permeability at about 6.5 wt%, and since natural electric resistance is high, the iron loss is made small. In this kind of steel sheets, if the Si content is 4.0 wt% or more, workability is abruptly worsened, and therefore it has been impossible to produce high Si steel sheets in industrial scales by the rolling process, but it has been found that the warm rolling could be performed on the thin steel sheets.
However, the warm rolling is restricted as that the high Si steel sheet is poor in bending processing and could not be welded at room temperatures. When the high Si steel sheet is going to warm-roll (200.degree. to 600.degree. C.) and if the rolls are cool, the steel strip passing therebetween is chilled and invites cracks by working. Therefore the rolls should be preheated.
It was found through the inventors' investigations that the high Si steel strip could be rolled efficiently with by reversings, and warming between each passes of the reverse rollings by means of the tension reels disposed within the warming furnaces, and further magnetic property could be satisfied therein by the recovery treatment between passes in the warming furnaces. In the rolling, the steel strip should be coiled completely on one of the tension reels in each step of the reverse rollings.